1. Field of the Invention
This invention relates to a network resistor unit suitable for use as a protection device for integrated circuit (IC) parts.
2. Description of the Prior Art
IC parts are being used increasingly as circuit-integration of electronic circuits and down-sizing of electronic equipments progresses. These IC parts are generally used together with protection resistors provided on the input side thereof as a countermeasure against static electricity, because the IC's can be destroyed easily by high voltage spikes such as static electricity from a human body, for example.
FIG. 4 is a sectional view of a conventional network resistor unit used as a protection resistor for IC parts as above, in which reference numeral 1 indicates a substrate, 2 indicates resistance films, 3 indicates electrodes, 4 indicates a common electrode, 5 indicates lead terminals, and 6 indicates a sheath.
In this drawing, a plurality of electrodes 3 and one common electrode 4 are formed on the substrate 1 made of an insulating material, such as ceramics or a phenol resin, and the resistance films 2 of certain width are formed with a certain spacing left therebetween so as to bridge each electrode 3 to the common electrode 4. These resistance films 2 are formed on the substrate 1 by the use of thick-film technique or thin-film technique and then set so as to have a desired resistance value by trimming. The common electrode 4 and respective electrodes 3 have corresponding lead terminals 5 secured by soldering, and the foregoing substrate 1, resistance films 2, electrodes 3 and 4, and lead terminals 5 are sealed by the sheath (casing) 6, except for a protruding portion of each lead terminal 5.
The thus produced network resistor unit is used as a protection resistor for IC parts (not shown) by soldering and securing the respective lead terminals 5 to a print-circuit board (not shown) which connects the resistive elements 2 to respective IC lead terminals and the common electrode 4 to a constant voltage point such as V+ or ground.
The foregoing conventional network resistor unit, however, became defective when static electricity of very high voltage was applied to some lead terminal 5 of the network resistor unit, because the resistance value of the resistance film 2 was permanently altered by the high voltage surge and thereafter it did not perform as a protection resistor of the desired resistance value.